Real-Time PET Imaging for Range Verification of Helium Radiotherapy

Abstract

Real-time range verification of particle beams is important for optimal exploitation of the tissue-sparing advantages of particle therapy. Positron Emission Tomography (PET) of the beam-induced positron emitters such as 15 O (T 1/2 = 122 s) and 11 C (T 1/2 = 1223 s) has been used for monitoring of therapy in both clinical and preclinical studies. However, the half-lives of these nuclides preclude prompt feedback, i.e., on a sub-second timescale, on dose delivery. The in vivo verification technique relying on the in-beam PET imaging of very short-lived positron emitters such as 12 N (T 1/2 = 11 ms), recently proposed and investigated in feasibility experiments with a proton beam, provides millimeter precision in range measurement a few tens of milliseconds after the start of an irradiation. With the increasing interest in helium therapy, it becomes relevant to study the feasibility of prompt feedback using PET also for helium beams. A recent study has demonstrated the production of very short-lived nuclides (T 1/2 = 10 ms attributed to 12 N and/or 13 O) during irradiation of water and graphite with helium ions. This work is aimed at investigating the range verification potential of imaging these very short-lived nuclides. PMMA targets were irradiated with a 90 AMeV 4 He pencil beam consisting of a series of pulses of 10 ms beam-on and 90 ms beam-off. Two modules of a modified Siemens Biograph mCT PET scanner (21 × 21 cm 2), installed 25 cm apart, were used to image the beam-induced PET activity during the beam-off periods. For the irradiation of PMMA, we identify the very short-lived activity earlier observed to be 12 N (T 1/2 = 11.0 ms). The range precision determined from the 12 N activity profile that is measured after just one beam pulse was found to be 9.0 and 4.1 mm (1σ) with 1.3 × 10 7 4 He ions per pulse and 6.6 × 10 7 4 He ions per pulse, respectively. When considering 4.0 × 10 7 4 He ions, which is about the intensity of the most intense distal layer spot in a helium therapy plan, a range verification precision in PMMA of 5.7 mm (1σ) can be realized. The range precision scales approximately with the inverse square root of the number of 4 He ions, i.e., the relative statistical accuracy of the number of coincidence events. Thus, when summing data over about 10 distal layer spots, this study shows good prospects for obtaining 1.8 mm (1σ) precision in range verification, within 50 ms after the start of a helium irradiation by in-beam PET imaging (scanner 29% solid angle) of 12 N.

Range Verification by Means of Prompt-Gamma Detection in Particle Therapy 1

“…This, in turn, opened a way to biological washout which blurred the image. Dendooven et al proposed to eliminate this obstacle by focusing on short-lived β + -emitters [17] and tested the experimental feasibility of such a solution [18]. A real breakthrough, however, came with the development of a high-acceptance, high-efficiency INSIDE setup currently operated at CNAO, Pavia [19].…”

Time of Flight in Perspective: Instrumental and Computational Aspects of Time Resolution in Positron Emission Tomography

“…Examples of limited-angle PET systems are breast scanners with integrated biopsy solutions and dedicated heart and prostate systems [160]. Other examples are helmet-type PET scanners for brain [161] and dual-panel systems for in-vivo dosimetry in particle therapy [162][163][164][165][166]. Unfortunately, reconstruction of non-TOF PET data with limited angular coverage (i.e.…”

“…Examples of limited-angle PET systems are breast scanners with integrated biopsy solutions and dedicated heart and prostate systems [ 159 ]. Other examples are helmet-type PET scanners for brain [ 160 ] and dual-panel systems for in-vivo dosimetry in particle therapy [ 161 ]–[ 165 ]. Unfortunately, reconstruction of non-TOF PET data with limited angular coverage (i.e., PET systems for which the local Tuy condition is not satisfied for all voxels in the FOV) suffer from strong artifacts, such as strong blurring in one direction [ 166 ], [ 167 ].…”

Radioactive Beams for Image-Guided Particle Therapy: The BARB Experiment at GSI

“…The detectors have been custom-modified such that they can be switched off and on with switching times of less than a millisecond. This switching has been essential in earlier work on nitrogen-12 positron imaging in proton and helium therapy, where short and intense beam pulses were used ( 51 , 52 ). Whether this option will be useful when imaging radioactive beams will depend on the time structure and intensity of the beams.…”